Enantiomers Enantiomeric separation/Enantioseparations

From the method development and robustness point of view, the temperature is a parameter that controls equilibria such as pK and enantiomer—chiral selector complexation, or induces structural changes in, e.g., proteins.For chiral separations, generally a lower temperature results in better enantioseparation, but even the opposite has been observed. Sometimes a raise in temperature does not so much affect the enantiomeric separation, but increases the resolution between an enantiomer and a matrix component. ... 

Enantiomeric resolution of solutes that fit within the molecular cavity, which is chiral, results in the formation of an inclusion complex (Ref. 169 and Fig. 4). In general, the enantiomers are separated on the basis of formation constants of the host-guest complexes. The enantiomer that forms the more stable complex has a greater migration time because of this effect. The chiral recognition mechanism for cyclodextrin enantioseparation has been discussed in several works (163-168). 

Discrimination between the enantiomers of a racemic mixture is a complex task in analytical sciences. Because enantiomers differ only in their structural orientation, and not in their physico-chemical properties, separation can only be achieved within an environment which is unichiral. Unichiral means that a counterpart of the race-mate to be separated consists of a pure enantiomeric form, or shows at least enrichment in one isomeric form. Discrimination or separation can be performed by a wide variety of adsorption techniques, e.g. chromatography in different modes and electrophoresis. As explained above, the enantioseparation of a racemate requires a non-racemic counterpart, and this can be presented in three different ways ... 

Efficient methods of enantioseparation are always required to control the enantiomeric purity, or to separate the target molecule or one of its chemical precursors (obtained from conventional synthetic procedures), or for monitoring the completion of enantioselective reaction process (since the production of single enantiomer is a real difficult task). 

The most important advantages of direct chromatographic separations of enantiomers for preparative purposes include the presence of the resolved enantiomers in high enantiomeric purity in different volume fractions of the mobile phase and no loss of chiral selectors and almost no risk of contamination of the desired enantiomers by the chiral selector. Parallel to enantioseparation, other impurities may be removed from chiral compounds during this process. 

The appearance of capillary electrophoresis in the mid-1980s coincided with a period of important expansion of enantioseparation techniques," and soon it was applied to the analysis of enantiomeric mixtures of the most diverse character." As in HPLC, a chiral selector, with the ability to recognize stereoselectively the enantiomers, has to be incorporated into the separative system. This CS is either added to the background electrolyte as an additional solute, acts as a chiral surfactant forming chiral micelles, or incorporated as a constituent of the chromatographic support, which is filling the capillary. The difference in mobilities observed for enantiomers is the result of the differentiated association between each one of them and the CS and/or the difference in mobility of the transient diastereomeric adsorbates formed.""... 

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